A long-term goal of our research is to understand the earliest features of autism, leading to the design of early diagnostic measures and potential targets for early treatment. Autism is rarely diagnosed before the third year of life, yet retrospective video studies of infants later diagnosed with autism indicate that symptoms are often present by the first birthday. Earlier identification and treatment require greater knowledge of the developmental trajectory of children at risk for autism. A main goal of this study is to define this trajectory, via a longitudinal study across the age span from 6 to 42 months. This application is linked to an identical application submitted by Dr. Marian Sigrnan of UCLA. Across the two sites (UC Davis & UCLA), the study will contrast the early development of children at risk for autism, infant siblings of children with autism (n=180), with two other groups of infants at less risk, those with older siblings with developmental delays (n=90), and those with older typically developing siblings (n=90). Due to the strong genetic influence in autism and based on previous studies, we expect that 3-6% of the infant siblings of children with autism will develop autism in the first three years of life. An additional 15-35% of them will develop the broader autism phenotype, consisting of features that are milder but qualitatively similar to autism. Past research on the broader phenotype has focused on older children and adults; symptoms in early childhood have not been studied. New data from an ongoing multinational pilot study of infant siblings (PI: Marian Sigman, one of the PI's on this linked application) demonstrate significant group differences in verbal and nonverbal communication. Given these and other data, the second main goal of this study is to define the developing broader autism phenotype across infancy and early childhood. Specific aims of the project are: 1) to compare the developmental course from 6 to 42 months of age of the three groups of infants, examining both social and communicative characteristics known to be associated with autism and the precursors of these characteristics; 2) to identify siblings who develop either the full syndrome of autism, or the broader autism phenotype, in the preschool period; and 3) to examine relations between measures of facial, emotional, and linguistic perception and social and communicative interaction at 6 and 18 months with later development of autism or the broader phenotype.